1. Field of the Invention
The present invention relates to optical collimators and methods for assembling them, and more particularly to an optical collimator which has simple structure and is easily assembled.
2. Description of Prior Art
Optical fibers are becoming increasingly important in many applications involving transmission of light. Over long distances, single-mode silica optical fibers are used in optical communications networks to transmit large amounts of data with low loss and with immunity from electromagnetic interference.
The light transmitted through the fiber can also be manipulated in a variety of ways such as filtering, modulation, splitting, and combining. In most cases, two or more fibers are fed into an enclosure encasing an optical system. Input light enters the enclosure via one or more fibers, and interacts with an optical device within the enclosure. The light resulting from the interaction exits the enclosure via one or more fibers. One example of such a two-port system is an optical isolator in which two polarizers sandwiching a Faraday rotator therebetween are positioned between two fibers that have collimating lenses at their free ends. The polarization angles are set such that light can propagate in one direction through the isolator, but is prevented from propagating in the opposite direction. Another example is a dielectric interference filter which transmits or reflects selected wavelengths.
In practice, the fibers used in such optical systems are typically held in collimator assemblies which are easily aligned to the enclosure of the optical system. A commercial collimator includes as its most fundamental components the fiber, a small glass tube (sometimes referred to as a capillary) which holds an exposed end of the fiber, and a graded index (GRIN) lens. A GRIN lens used with optical fibers is a generally cylindrical piece of optical glass having a length greater than its diameter. It is fabricated to have a radially varying index of refraction that is greater toward a center thereof. Therefore, it produces a focusing effect similar to that of a conventional convex lens. Both the fiber and the GRIN lens are inserted and held in the glass tube. Thus the GRIN lens collimates light diverging from the smaller core of the optical fiber, or focuses light to the smaller core.
U.S. Pat. No. 6,168,319 discloses a collimator 100 illustrated in cross-sectional view in FIG. 1. An optical fiber 112 is received within a central axial aperture of a glass capillary 114. The fiber 112 typically includes a silica-based core, a cladding, and a surrounding polymeric coating such as that of an acrylate material. The polymeric coating is stripped from the fiber 112 at a portion thereof to be received by the capillary 114. For strength and thermal stability, the polymeric coating extends into a conical aperture (not labeled) defined in an end of the capillary 114 but does not touch the capillary 114 itself. As a result, the capillary 114 typically holds only the core and cladding of the fiber 112. Additional adhesive may be applied to strengthen the joint between the fiber 112 and the capillary 114.
The capillary 114 is slidably fitted within a central bore of a glass tube 118 snugly but easily, and is eventually bonded to the glass tube 118. A GRIN lens 120 also fits within the bore of the glass tube 118, and is bonded to the glass tube 118. The glass tube 118 encasing the capillary 114 and the GRIN lens 120 is held by an aligning apparatus. The aligning apparatus is adjusted to accurately align the capillary 114 and the GRIN lens 120. After alignment, the glass tube 118 is fitted into and bonded to a stainless steel tube 119.
Alignment of the capillary 114 and the GRIN lens 120 must be performed in the glass tube 118, and the glass tube 118 is subsequently encased in and protected by the stainless steel tube 119. The required number of components is unduly great, and complicates the assembly process. Furthermore, in order to adjust the capillary 114 and the GRIN lens 120 to be in accurate alignment, a gap between an inner end of the capillary 114 and an inner faceted end of the GRIN lens 120 must be provided to prevent the ends from rubbing against each other. Therefore, a length of the GRIN lens 120 must be less than 0.25 pitch, to ensure that a focal point of the GRIN lens 120 is beyond the inner faceted end thereof. A GRIN lens 120 having a length of 0.25 pitch cannot be used in the optical collimator 100.
Thus, it is desired to provide an optical collimator which has relatively few components and which can be easily assembled.
An object of the present invention is to provide an optical collimator having relatively few components.
Another object of the present invention is to provide an optical collimator which can be assembled easily.
To achieve the above objects, the present invention provides an optical collimator and a method of assembling the collimator. The collimator comprises an optical fiber, a capillary, a GRIN lens and a glass tube. A bore is defined in the capillary. A conical depression is defined in an end of the capillary and in communication with the bore. The optical fiber is retained in the bore of the capillary. An end of the capillary is bonded with an end of the GRIN lens. The capillary and the GRIN lens are encased in the glass tube.
Other objects, advantages and novel features of the present invention will be drawn from the following detailed description of embodiments of the present invention with attached drawings, in which: